Showing papers by "Prashant V. Kamat published in 1993"

Electrochemically assisted photocatalysis: titania particulate film electrodes for photocatalytic degradation of 4-chlorophenol

Abstract: A commercially available TiO[sub 2] powder (Degussa P25) has been used to prepare thin particulate films on conducting glass plates. These semiconductor particulate films are photoelectrochemically active with properties similar to an n-type semiconductor. The recombination between the photogenerated charge carriers can be suppressed by applying an external anodic bias. Electron scavengers, such as oxygen, affect the photocurrent generation by competing for the photogenerated electrons. These particulate films provide a convenient method for accelerating the photocatalytic reaction by applying an external bias. For example, the rate of photocatalytic degradation of 4-chlorophenol greatly increases when the TiO[sub 2] particulate film electrode is maintained at an external anodic bias (0.6 V vs SCE) during the UV photolysis. 26 refs., 6 figs.

Photoelectrochemical behavior of thin CdSe and coupled TiO2/CdSe semiconductor films

Abstract: Photoelectrochemical effects at CdSe thin film electrodes (thickness 30-200 [angstrom]) have been investigated by monitoring open-circuit voltage and short-circuit current at varying film thicknesses and incident light intensities. Unlike the situation in conventional photoelectrochemical cells that employ single-crystal or polycrystalline semiconductors, the charge separation in thin CdSe films is not controlled by the space charge layer at the semiconductor-electrolyte interface, but it is controlled by the differing rates of electron and hole transfer into the solution. Quick decay of the photocurrent, even in the presence of a redox couple such as [Fe(CN)[sub 6]][sup 3[minus]/4[minus]], suggests high degree of charge recombination within the thin CdSe film. It is possible to improve the photocurrent stability of a thin CdSe film by coupling it with a TiO[sub 2] particulate film. The improved charge separation in the coupled semiconductor system has a beneficial effect in improving the photocurrent stability of thin semiconductor films. 22 refs., 8 figs., 1 tab.

Photoelectrochemistry of Quantized WO3 Colloids: Electron Storage, Electrochromic, and Photoelectrochromic Effects

Abstract: Electron storage effects in quantized WO[sub 3] colloids have been investigated by spectroelectrochemical and photochemical methods. Electrons trapped within the colloidal particles exhibit blue coloration with absorption in the red-IR region. From picosecond laser flash photolysis experiments, we estimate the rate constant for electron trapping to be 10[sup 10] s[sup [minus]1]. These trapped electrons are stable in an inert atmosphere and can be utilized to reduce substrates such as thiazine and oxazine dyes which have reduction potentials less negative than the conduction band of WO[sub 3]. The rate constants for the heterogeneous electron transfer at the semiconductor/electrolyte interface are in the range (0.7-2.4) [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1]. 43 refs., 10 figs., 1 tab.

Aggregates of fullerene C60 and C70 formed at the gas-water interface and in DMSO/water mixed solvents. A spectral study

Abstract: The absorption spectra of pure spread films of C 60 and C 70 as well as mixtures of C 60 or C 70 with 1,2-dioleoyl-sn-glycerol, DOG (C 60 /DOG or C 70 /DOG=1/1, 1/4, 1/20), have been measured at the nitrogen-water interface These spectra resemble those of C 60 and C 70 solid films rather than spectra reported in nonaqueous solvents The data suggest that aggregates of fullerene molecules form in spread films on the water surface Even in C 60 /DOG (or C 70 /DOG) 1/20 mixed spread films, the fullerene molecules exist in microdomains rather than homogeneous 2-dimensional solutions Aggregate formation for C 60 and C 70 in DMSO/water mixed solvents has also been investigated

Photochemistry of squaraine dyes. 5. Aggregation of bis(2,4-dihydroxyphenyl)squaraine and bis(2,4,6-trihydroxyphenyl)squaraine and their photodissociation in acetonitrile solutions

Abstract: The equilibrium constants and thermodynamic parameters for aggregate formation of bis(2,4-dihydroxyphenyl)- squaraine and bis(2,4,6-trihydroxyphenyl)squaraine have been studied by absorption spectroscopy. The values of the equilibrium constant, K, for the dimerization of SQ1 and SQ2 in acetonitrile at 300 K are (3.1 ± 0.3) × 10 5 M -1 and (2.2 ± 0.1) × 10 5 M -1 , respectively. Iodine enhances the aggregation process. This has been attributed to the formation of charge-transfer complexes between iodine and the dyes and the stronger tendency of these complexes to form aggregates. Excitation of the squaraine dimer with a 532-nm laser pulse leads to the formation of the excited singlet state of monomeric dye as the excited dimer dissociates within the laser pulse duration of 18 ps.

Radical adducts of fullerenes C60 and C70 studied by laser flash photolysis and pulse radiolysis

Abstract: The reaction of organic radicals (R[sup [sm bullet]] = benzyl, tert-butyl, trichloromethyl) with C[sub 60] and C[sub 70] has been investigated using laser flash photolysis and pulse radiolysis techniques. The alkyl radicals are very reactive toward fullerenes and form radical adducts, (RC[sub 60])[sup [sm bullet]] and (RC[sub 70])[sup [sm bullet]]. The difference absorption spectrum of (RC[sub 60][sup [sm bullet]]) exhibits broad absorption in the visible, while that of (RC[sub 70])[sup [sm bullet]] exhibits bleaching of absorption in the region 440-560 nm. The rate constants for the reaction of these radicals with fullerenes are in the range of 10[sup 8] M[sup [minus]1] s[sup [minus]1] for dichloromethyl to diffusion controlled for tert-butyl radicals. 30 refs., 6 figs., 1 tab.

Photochemistry of squaraine dyes. 6. Solvent hydrogen bonding effects on the photophysical properties of bis(benzothiazolylidene)squaraines

Abstract: The photophysical properties of two bis(benzothiazolylidene)squaraine dyes have been studied in aromatic hydrocarbon and alcoholic solvents. These dyes exhibit fluorescence quantum yields of 0.06-0.52 and excited singlet lifetimes of 0.48-2.5 ns in these solvents. Although the photophysical properties of the dyes are independent of solvent polarity in the hydrocarbon solvents, in alcoholic solvents a marked hyposchromic shift in the absorption and emission spectra and a reduction in lifetimes of excited state have been observed. These effects have been correlated with the hydrogen bond donating ability of the alcoholic solvents. 17 refs., 6 figs., 2 tabs.

Excited-state behavior and one-electron reduction of fullerene C60 in aqueous .gamma.-cyclodextrin solution

Abstract: Excited-state properties and one-electron reduction of C 60 in aqueous γ-cyclodextrin (γ-CD) solutions have been studied by laser flash photolysis and pulse radiolysis techniques. The difference absorption spectrum of C 60 - anion radical in aqueous γ-CD solution has been characterized by reducing C 60 with radiolytically produced radicals (e aq - , (CH 3 ) 2 COH or MV .+ )

Excited-state properties and photosensitization behaviour of bis(2,4-dihydroxyphenyl)squaraine

Abstract: Two protonation equilibria (pKa at 3.7 and 7.4) have been identified for the dye, bis(2,4-dihydroxyphenyl)-squaraine in the ground state. These various protonation forms exhibit widely different absorption properties. The singlet excited-state behaviour of this dye has been characterized by picosecond laser flash photolysis. The fluorescence quantum yield and the excited singlet lifetime of the unprotonated dye (ϕf= 0.092, τs= 740 ps) are considerably higher than that of the neutral (ϕf= 0.037, τs= 130 ps) or the protonated (ϕf= 0.01, τs⩽ 100 ps) forms of the dye. The triplet excited state has been characterized by both direct excitation and T–T energy transfer methods. The triplet excited state has an absorption maximum around 430 nm and a lifetime of ca. 100 µs in ethanol. Photosensitization of a thin microporous TiO2 semiconductor film has been carried out by adsorbing the squaraine dye from an ethanolic solution and exciting it with visible light. An incident photon-to-current conversion efficiency of 0.05% at 500 nm has been obtained for the dye-modified TiO2 electrode.

Surface Modification of CdS Colloids with Mercaptoethylamine

Abstract: Surface modification as well as size control of CdS colloids in acetonitrile has been achieved with mercaptoethylamine (MEA). The CdS emission yield increases as it interacts with the surface modifier. A decrease in the hole-trapping yield was observed in optically excited CdS colloids which were modified with MEA. Use of such surface modifiers in bridging a semiconductor colloid with a gold colloid has also been demonstrated.

GaAs Quantum Dots

Abstract: GaAs quantum dots have been prepared by reacting gallium acetylacetonate with trimethylsilylarsine in triethylene glycol ether at 216 K. The colloidal dispersion of GaAs nanocrystals has been characterized by transmission electron microscopy, electron diffraction, ultrafiltration, optical absorption and photoluminescence spectroscopy, and transient pump-probe spectroscopy; the spectral data were also fitted to a model that accounts for the size-dependent optical properties of a Gaussian distribution of particle sizes. Our results indicate that the spectral region above 470 nm can be assigned solely to GaAs nanocrystals having a mean diameter of 40.0 A with a standard deviation of 9.7 A. The origin of spectral features below 470 nm has not yet been determined conclusively.

Photoinduced charge transfer processes in ultrasmall semiconductor clusters. Photophysical properties of CdS clusters in Nafion membrane

Abstract: The photophysical properties of quantized CdS clusters in a perfluorosulfonate polymer (Nafion) film have been investigated by time-resolved emission spectroscopy. The ultrasmall CdS clusters were prepared by exposing Cd2+-exchanged polymer film to H2S. Size-dependent absorption and emission properties were observed during the growth of these clusters. The emission decay is multiexponential with lifetimes ranging from 0.85 to 480 ns.

Photosensitizing properties of squaraine dyes

Abstract: The present report summarises our results on the photophysical and photochemical investigations of a series of squaraine dyes which exhibit intense and sharp absorption bands in the visible and near infrared regions. The intramolecular charge-transfer transitions arising from the “donor-acceptor-donor” arrangements of these dyes have an interesting effect on their excited state properties. The major nonradiative decay process of squaraines is by rotation about the C-C bonds between the central cyclobutane unit and its neighbouring phenyl groups. Microencaging of one of the dyes by Β-cyclodextrin or poly(4-vinylpyridine) was found to restrict this motion, bringing about up to 90-fold enhancement in its fluorescence yield. These aspects as well as the dynamics of charge transfer from the excited singlet state of some of the squaraine dyes to TiO2 and the recombination of the injected charge with the dye radical cation are discussed.

A new photodegradable polyamide containing o-nitrobenzyl chromophore. Steady state and laser flash photolysis studies

Abstract: A new photodegradable and thermally stable polyamide containing a photosensitive o-nitrobenzyl chromophore (4) was prepared by melt condensation of 4-α-methylamino-3-nitro-p-toluoyl chloride (5). On photolysis, 4 undergoes efficient photodegradation and becomes soluble in methanol and aqueous base, whereas the unirradiated polymer is insoluble in both solvents. This solubility difference renders the polymer 4 a potential material for microelectronic applications. The progress of photodegradation can be monitored by measuring the inherent viscosity of the polymer as a function of the irradiation time. The photodegradation pathway of 4 was studied by examining both steady state and laser flash photolysis of a low-molecular-weight model compound, 4-N-benzoyl-α-methylamino- 3-nitro-N′,N′-dimethyl-p-tolumide (6). Steady state irradiation of 6 in methanol gives a mixture of N-methyl-benzamide (10) and an azobenzenedicarboxylic acid 8. The transient species with an absorption maximum around 440 nm, produced from 6 on photolysis using 20 ps flashes of 355 nm laser light or 10 ns flashes of 308 nm laser light, is attributed to the nitronic acid 12. The formation of 12 is suggested to occur through a very fast singlet route and a slower triplet route.

Visible Laser-Induced Oxidation of C70 on Titanium Dioxide Particles.

Abstract: Visible laser-induced degradation of C70 is observed on titanium dioxide particles; diffuse reflectance laser flash-photolysis experiments, which highlight the photochemical oxidation of C70 on TiO2 particles, are described.